It is often necessary, or at least desirable, to concentrate a liquid mixture by removing a portion of the solvent, generally water, from the liquid mixture. The resulting product, therefore, is in a more concentrated form. It has been common to concentrate fruit and vegetable juices such as orange juice, grapefruit juice, grape juice, and tomato juice by evaporation to remove water. In addition, seawater and brackish water have been concentrated by evaporation, although the condensed vapor has been recovered as usable potable water rather than discarded as in concentrating fruit and vegetable juices. Nevertheless, each is a concentrating process. In the case of juice, the concentrate is the desirable product; whereas, in obtaining potable water from seawater or brackish water the concentrate is discarded.
Evaporative concentration as described, as well as evaporation of chemical solutions or liquid dispersions, requires substantial energy since it relies on the latent heat of vaporization. Scaling of equipment and enhanced corrosion are often inherent at the temperatures involved in evaporative concentration. Loss of flavor and aroma also result during evaporative concentration of food products.
Because of the shortcomings involved in evaporative concentration, it has been found advantageous to freeze concentrate many products, particularly those having water as the liquid carrier. Generally, reduced energy is required since freeze concentrating relies on the heat of fusion instead of the heat of evaporation. In such a process, water is removed by first producing ice crystals which are then separated from the concentrate. Next, the ice crystals are washed to remove the remaining concentrate from them. The ice crystals can then be discarded, or melted if potable water is desired.
Various types of apparatus are used to cool the liquid mixture to form crystals of the solvent, including shell and tube heat exchangers, sometimes called freeze exchangers. Regardless of the apparatus used, the resulting liquid containing the crystals is generally transferred to a separate settling tank in which the crystals are separated by gravity from the liquid. The crystals, being less dense than the solvent or liquid, rise to the top. The resulting slurry is then transferred to another tank for washing and subsequent melting if the object is to obtain pure liquid from the process. While there are variations on the described system, they share the disadvantage of employing more equipment and piping than is desired and which can be made partially or totally ineffective by frozen liquid sticking or adhering to internal surfaces.
In systems which use a shell and tube freeze exchanger, it is generally impractical to freeze out more than a small amount of liquid in a single pass through the heat exchanger. To avoid wasting the energy used to cool the liquid, the liquid is advisably recirculated through the freeze exchanger. However, it is generally desirable to remove at least some of the frozen liquid crystals before the liquid is recirculated to prevent the slurry solids concentration from reaching a level which makes it difficult to pump. In addition, removal of the crystals makes it possible for more liquid to be recirculated and thus cooled in the freeze exchanger.
From the above it is believed clear that a need exists for an improved apparatus and method for removing frozen crystals from a liquid mixture so that the liquid can be recirculated, and the separated crystals washed, in a single vessel.